Pike hangs on exit when compiled --with-cleanup-on-exit.

Imported from https://youtrack.roxen.com/issue/PIKE-78
Reported by @grubba
LysLysKOM 22448381:
22448381 igår 06:32 /23 rader/ Martin Nilsson (Coppermist)
Mottagare: Henrik Grubbström (Lysator) <15078>
Mottagare: Martin Nilsson (Coppermist) <16144>
Ärende: trådlås
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Pike kompilerad med --with-dmalloc hänger i exit.

(gdb) bt
1. 0  __lll_lock_wait () at
../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
1. 1  0x00007ffff7232b70 in pthread_cond_broadcast@@GLIBC_2.3.2 ()
    at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_broadcast.S:133
1. 2  0x00000000005728bd in unlock_pike_compiler ()
    at /home/nilsson/pike/src/pike_compiler.cmod:69
1. 3  0x00000000005d7b1a in exit_threads_disable (o_UNUSED=0x0)
    at /home/nilsson/pike/src/threads.c:1028
1. 4  exit_interleave_mutex (im# im@entry
0xa4fa20 <mc_mutex>)
    at /home/nilsson/pike/src/threads.c:1086
1. 5  0x00000000004d116a in exit_mc () at
/home/nilsson/pike/src/gc.c:6342
1. 6  0x0000000000683733 in exit_builtin_modules ()
    at /home/nilsson/pike/src/pike_modules.c:137
1. 7  0x0000000000684d92 in exit_modules ()
    at /home/nilsson/pike/src/pike_modules.c:533
1. 8  0x000000000051cc28 in pike_do_exit (num=0)
    at /home/nilsson/pike/src/pike_embed.c:436
1. 9  0x0000000000414a22 in main (argc# 2, argv
0x7fffffffe3f8)
    at /home/nilsson/pike/src/main.c:682
(22448381) /Martin Nilsson (Coppermist)/------------